4-Phenylbutyric acid treatment rescues trafficking and processing of a mutant surfactant protein-C.

Mutations in the SFTPC gene, encoding surfactant protein-C (SP-C), are associated with interstitial lung disease (ILD). Knowledge of the intracellular fate of mutant SP-C is essential in the design of therapies to correct trafficking/processing of the proprotein, and to prevent the formation of cytotoxic aggregates. We assessed the potential of a chemical chaperone to correct the trafficking and processing of three disease-associated mutant SP-C proteins. HEK293 cells were stably transfected with wild-type (SP-C-WT) or mutant (SP-C-L188Q, SP-C-Delta exon4, or SP-C-I73T) SP-C, and cell lines with a similar expression of SP-C mRNA were identified. The effects of the chemical chaperone 4-phenylbutyric acid (PBA) and lysosomotropic drugs on intracellular trafficking to the endolysosomal pathway and the subsequent conversion of SP-C proprotein to mature peptide were assessed. Despite comparable SP-C mRNA expression, proprotein concentrations varied greatly: SP-C-I73T was more abundant than SP-C-WT and was localized to the cell surface, whereas SP-C-Delta exon4 was barely detectable. In contrast, SP-C-L188Q and SP-C-WT proprotein concentrations were comparable, and a small amount of SP-C-L188Q was localized to the endolysosomal pathway. PBA treatment restored the trafficking and processing of SP-C-L188Q to SP-C-WT concentrations, but did not correct the mistrafficking of SP-C-I73T or rescue SP-C-Delta exon4. PBA treatment also promoted the aggregation of SP-C proproteins, including SP-C-L188Q. This study provides proof of the principle that a chemical chaperone can correct the mistrafficking and processing of a disease-associated mutant SP-C proprotein.